Recently, the thermal printing head, which has advantages such as a low noise, a low maintenance cost and a low running cost, is extensively used for thermal recorders of various types of OA equipment such as a facsimile or stencil printing.
Generally, the thermal printing head has the following structure. Specifically, a glaze glass layer is formed on an alumina substrate, a heat-generating resistor layer and a conductive layer such as aluminum are formed on the glaze glass layer, and a heat-generating resistor and an electrode are formed by a photoengraving process. Besides, it has a structure in which a protective layer for protecting the heat-generating resistor and the electrode is formed by a film forming technology such as a sputtering process. And, in order to prevent oxygen from dispersing from the glaze glass layer to the heat-generating resistor and to retain the properties of the heat-generating resistor, a primer layer for a heat-generating resistor may be additionally formed between the glaze glass layer and the heat-generating resistor formed on the alumina substrate.
The protective layer and the primer layer for the heat-generating resistor are formed by the film forming technology such as the sputtering process using, for example, powder of a sinter comprising silicon nitride and silicon dioxide as a target, but a foreign matter (splash) may be captured into the protective layer and the primer layer for the heat-generating resistor while the protective layer and the primary layer for the heat-generating resistor are formed. The foreign matter includes floating dust present in the atmosphere where the thermal printing head is produced or dust dispersed from the production machine, human bodies or the like and adhered to the top/bottom layers and into the layers of the protective layer for the thermal printing head and the primary layer for the heat-generating resistor. Especially, the sinter, which is used as the target, is a problem as the cause to produce the foreign matter.
Specifically, the thermal printing head manufacturing environment is designed to keep higher cleanness as compared with conventional cleanness, owing to the progress of the cleaning technology on a cleaning room and the like, and the floating dust and the dust from the manufacturing machine and the human body are mostly removed. However, when the protective layer and the primary layer for the heat-generating resistor are formed by the sputtering process or the like, the foreign matter which is dispersed from the sinter used as the target to adhere to the protective layer and the primary layer for the heat-generating resistor is hardly removed. Therefore, the foreign matter dispersed from the sinter adheres to the protective layer and the primary layer for the heat-generating resistor. The protective layer and the primary layer for the heat-generating resistor capture the foreign matter. The foreign matter is produced from the sinter used as the target because a difference in size of particles forming the sinter and a partial difference in density of the sinter due to pores cause partial deviation of atoms, which are consumed from the sinter when the film is formed, to form fine irregularities on the surface of the sinter, and especially to separate a part of the sinter due to an abnormal electric discharge produced on the protruded portions. In other words, a portion of the separated sinter disperses as the foreign matter from the sinter to adhere to the protective layer and the primary layer for the heat-generating resistor.
Thus, in the thermal printing head containing a part of the sinter as the foreign matter in the protective layer, the foreign matter comes in contact with a recording medium and is separated from the protective layer, causing a pinhole at he position of the protective layer where the foreign matter s separated. And, if the separated foreign matter were captured into the periphery of the separated position, the protective layer of the pertinent periphery is broken.
Moisture, a corrosive substance or the like is easy to enter into the pinhole and the broken part of the protective layer formed as described above to cause a problem that the electrode or the heat-generating resistor is corroded or deteriorated, resulting in impairing the properties of the thermal printing head.
And, if the foreign matter were captured into the primary layer for the heat-generating resistor, the heat-generating resistor layer and the like formed on the primary layer for the heat-generating resistor particularly suffers from a defect and has drawbacks that the heat-generating resistor has a defect in its property such as a resistance value and its quality such as service life.
In addition, there was a problem that quality of an image printed on a printing medium by a recorder was degraded because quality such as the feature or service life of the thermal printing head mounted on the recorder was affected.
The present invention was completed to remedy the above-described problems, and aims to provide a thermal printing head which reduces pinholes to be formed in the protective layer and the primary layer for the heat-generating resistor or a breakage to be caused in the protective layer and the primary layer for the heat-generating resistor and has an excellent environmental resistance, high reliability and a high quality level.
It is also an object of the present invention to provide a thermal printing head, which retains the properties and quality of the heat-generating resister and has a high reliability and a high quality level.
It is another object of the present invention to provide a process for producing a thermal printing head which reduces a pinhole formed in the protective layer or a breakage caused in the protective layer and has excellent environmental resistance, high reliability and a high quality level.
It is still an object of the present invention to provide a process for producing a thermal printing head, which retains the properties and quality of the heat-generating resister and has high reliability and a high quality level.
Furthermore, it is an object of the present invention to provide a recorder which mounts a thermal printing head, which retains the properties and quality of the heat-generating resister and has high reliability and a high quality level, and stably outputs a high-quality recorded image.
Besides, it is an object of the present invention to provide a recorder which mounts a thermal printing head, which reduces a pinhole to be formed in the protective layer or a breakage to be caused in the protective layer and has an excellent environmental resistance, high reliability and a high quality level, and stably outputs a high-quality recorded image.
In addition, it is an object of the invention to provide a sinter which can be used suitably to form the protective layer or the primary layer for the heat-generating resistor of the thermal printing head.
And, it is also an object of the invention to provide a target which can be used suitably to form the protective layer or the primary layer for the heat-generating resistor of the thermal printing head.